refactor: Delete ChainstateManager::IsSnapshotActive() method

IsSnapshotActive() method is only called one place outside of tests and
asserts, and is confusing because it returns true even after the snapshot is
fully validated.

The documentation which said this "implies that a background validation
chainstate is also in use" is also incorrect, because after the snapshot is
validated, the background chainstate gets disabled and IsUsable() would return
false.

src/validation.h

@@ -1189,10 +1189,6 @@ public:
      */
     mutable VersionBitsCache m_versionbitscache;
 
-    //! @returns true if a snapshot-based chainstate is in use. Also implies
-    //!          that a background validation chainstate is also in use.
-    bool IsSnapshotActive() const;
-
     std::optional<uint256> SnapshotBlockhash() const;
 
     //! Is there a snapshot in use and has it been fully validated?